Reliability

As long as mail_fsync is not set to "never", any (local) filesystem should be equally reliable from data loss issues from dovecot usage. This is because dovecot (and most SMTP-servers) will not aknowledge a message as processed before fsync() returns sucessfully. To quote Timo:

"Success isn't returned to dovecot-lda or IMAP APPEND call until the mail has been fsynced. As long as the disk doesn't lie and the filesystem doesn't lie, there is zero data loss when fsyncing isn't disabled with Dovecot."

What about NFS? Cluster filesystems? Running with mail_fsync=never ?

Performance

Dovecot typically runs on Maildirs containing many small files in each directory, and needs high rate of random IOPS. When selecting filesystem for dovecot, one should look for a filesystem that can handle many small files, over filesystems that can efficiently stream large files.

Separate filesystems for maildirs and indexes? Cluster/NFS filesystem for maildirs, local filesystem for indexes ?